In recent years, nickel-titanium alloy materials which are the compositions to generate shape-memory effect or superelasticity effect have been used as materials for various kinds of industrial products for utilizing their functionality. In the development of these products, however, the fact that the nickel-titanium alloy materials have poor workability constitutes a big obstacle. For this reason, dissimilar metal joint products of nickel-titanium alloy material with pure titanium material are highly evaluated in the uses as a functional material because the pure titanium material shows good workability and has functionality such as biocompatibility and corrosion resistance.
In addition, in the dissimilar metal joint products of nickel-titanium alloy material with pure titanium material, the nickel-titanium alloy material gives higher tensile strength than that of the pure titanium material. Accordingly, the joint therebetween is required to have a tensile strength equal to or higher than that of the pure titanium material before joining.
Meanwhile, regarding the method for joining a nickel-titanium alloy material and a dissimilar metal material, since the joining by an ordinary welding method is difficult, generally there are adopted methods such as a mechanical joining method including swaging, and a method of plating the nickel-titanium alloy material, followed by brazing.
The dissimilar metal joint product of a nickel-titanium alloy material with a pure titanium material joined by a conventional ordinary method has, however, a problem that the joint shows a significantly lower tensile strength than that of the pure titanium material before joining. Furthermore, the dissimilar metal joint product of a nickel-titanium alloy material with a pure titanium material joined by brazing has a problem of having a wide heat-affected zone after joining.
In this respect, there has been proposed a technology described in “Method for Brazing Superelastic Alloy” of Patent Document 1, in which a superelastic alloy is covered with a titanium coating layer by sputtering or the like, and then brazing is applied to the titanium coating using a titanium alloy brazing material. The technology can provide a dissimilar metal joint product having high strength at the joint by covering the superelastic alloy with the titanium coating layer having high adhesion strength, compared with that of the products joined by a conventional ordinary method. Since, however, the technology conducts joining by brazing, the problem of having a wide heat-affected zone after joining cannot be solved.
Further, Patent Document 2 and Patent Document 3 proposed the respective technologies of joining a nickel-titanium alloy material and a nickel-base alloy material, and Patent Document 4 proposed a technology of joining a nickel-titanium alloy material and a dissimilar metal material.
Compared with the dissimilar metal joint products prepared by a conventional ordinary method, the above-described technologies can attain dissimilar metal joint products having higher strength at the joint through the formation of a metal structure solidified in a state where a pressing force is transmitted to the melt, or through the formation of a liquid metal forging structure in a thickness of about 20 μm or less over the whole area of the joint interface. In addition, pressurizing the joining materials at high temperatures and placing a heat-releasing zone in the vicinity of the joining face make it possible to obtain a dissimilar metal joint product having a narrow heat-affected zone after joining.
These technologies, however, caused problems such that the precision required in adjusting the joining face condition and the contact condition becomes insufficient and the liquid metal forging structure becomes difficult to be formed at a thickness of about 20 μm or less over the whole area of the joining face when the joining area increases, which resulted in easily causing deterioration of the strength and easily causing variation in the strength, at the joint. Consequently, the dissimilar metal joint products of nickel-titanium alloy material and pure titanium material according to the above-described technologies failed to attain the tensile strength at the joint equal to or higher than that of the pure titanium material before joining, as reported by Examples of Patent Document 4 as not less than 20 kg/mm2 and smaller than 30 kg/mm2 of tensile strength at the joint.    Patent Document 1: Japanese Patent No. 1968308    Patent Document 2: Japanese Patent No. 2516447    Patent Document 3: Japanese Patent No. 2563843    Patent Document 4: Japanese Patent No. 2737817